Note: Descriptions are shown in the official language in which they were submitted.
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METHODS FOR MANUFACTURING PACKAGING BOARD
The object of the invention is a method for manufacturing packaging board, in
which a board base of paperboard or cardboal-d is provided with at least one
silicon-
based, liquid-tight and gas-tight layer of coating. Another object of the
invention is
a method based on the coating of paper or a board base to manufacture liquid-
tight
and gas-tight packages, and products provided by using the methods, including
foodstuff packages and trays.
In order to be useful, for packages of liquid and other wet foodstuffs or
foodstuffs
which spoil easily the board or the paper must be provided with a liquid-tight
and
gas-tight coating. The coating prevents the oxygen in the air from penetrating
the
package and spoiling the product, and it also prevents the package from
getting wet
and the aromas of the product escaping from the package. Corresponding gas
tightness can be required from medicine, cosmetics, and detergent packages.
An effective way to render liquid packages, such as juice containers, liquid-
tight
and gas-tight is to provide the board of the container with a thin aluminium
foil.
Aluminium as such has also been used for peelable covers of yoghurt and
curdled
milk cups and butter and margarine boxes. However, aluminium foil has dis-
advantages: high manufacturing costs, it is not biologically decomposable,
there are
difficulties in regenerating the packaging material, and the package cannot be
heated
in a microwave oven. Another problem with detachable aluminium covers is that
they tear and burst easily.
An alternative solution for tightening the board or the paper used for
packages is to
provide it with one or more layers of polymeric coating. The number of layers
and
the material used depend on the requirements set by the packaged product. The
best
coating materials have essentially reached a tightness corresponding to that
of
aluminium foil and, as substitutive materials, they have eliminated the above-
mentioned disadvantages connected with aluminium. However, it has been
necessary to combine various polymeric materials in these substitutive
solutions so
that they comprise, for example, an oxygen-tight, water vapor-tight, and aroma-
tight
barrier layer, heat-seal layers on both sides of the paper or the board, and
one or
more layers of binding material to bind the polymers to the paper or the board
and
to each other. Therefore, the structure of the packaging paper or board
becomes
complex and the consumption of polymeric material is extensive.
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Examples of packages tightened according to the above description include
containers which are intended to be used as packages of milk, cream, sour
milk,
juice or other similar, liquid foodstuffs and which are entirely made of boal-
d
provided with layers of polymeric coating. In these containers, the board is
typically
provided with four or even five layers of polymeric coating so that, for
example, the
board comprises an oxygen-tight and aroma-tight barner of, e.g., polyamide, a
layer
of binding material on top of that, and, at the very top, a heat-seal layer of
polyethylene, for example, and another heat-seal layer of polyethylene is
provided
on the opposite side of the board. Another typical package application is a
portion
l0 package of, for example, milk, curdled milk, yoghurt, water, juice,
desserts or ice-
cream, in which the package is in the form of a small cup or a container, is
typically
made of plastic or coated paperboard, and is provided with a heat-sealed,
peelable
cover. The cover material is paper which is coated with an oxygen-tight and
aroma-
tight barrier consisting of, for example, polyamide, ethylene vinyl alcohol
copolymer (EVOH) or polyethylene terephtalate (PET), with a layer of binding
material, and with a heat-seal layer which is against the mouth of the
container or
the cup and which consists of, for example, styrene-modified copolymer of
ethylene
and methacrylic acid, making the product both heat-sealable and easy to be
peeled
off. Cosmetic products and pharmaceutical pills have been packed in a similar
manner, using plastic or glass containers provided with a peelable paper cover
which is sealed with a polymeric coating.
Patent publication US 5 340 620 describes paperboard provided with a silicon-
based
polymeric coating, in which the polymer serves as an oxygen-tight barrier.
According to the publication, the coating is provided by polymerizing
organosilane
by using UV irradiation, whereby, in addition to an inorganic polymeric
backbone,
organic bonds are formed in the coating when the organic groups of the silane
react
with each other. However, the portion of the inorganic polymeric backbone is
prevalent in the coating, which is why it can be too fragile to withstand, for
example, the creasing which is part of the manufacture of paperboard or
cardboard
containers; fiuthermore, there is no mention of the water vapor-tightness of
the
coating. It is obvious that the coating material of the embodiment of the
publication
cannot provide paperboard or cardboard suitable for liquid packages.
Moreover',
organosilanes are an expensive raw material for the coating.
Silicon-based coatings have also been described e.g. in the published
applications
3 5 DE 4 020 316 and 4 025 215, which cite paper as one possible substrate of
the
coating but which describe in detail the coating of plastic or metal only, and
according to the publications, the purpose of the coating is to provide
resistance to
wear so that the film-like substrate still maintains its flexibility.
Therefore, the
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publications are not concerned with packaging technology which is the object
of the
present invention.
Another use of tightened packaging board are foodstuff underlayers, such as
ovenable microwave or conventional oven trays which can be part of consumer
packages of foodstuffs, such as casserole foods intended to be heated, or
which can
be sold as separate products. Such underlayers must be impermeable to water
and
grease; and in addition to this, sufficient heat-resistance is required from
ovenable
trays. Polyester coated paperboard has been used in oven trays; however, its
disadvantages include the thickness of the required polymeric layer and the
fact that
it is very difficult for the polymeric coating to withstand typical oven
temperatures
of more than 200 °C. Polypropylene has been used as the polymer coating
in
microwave ovenable trays.
The purpose of the invention is to present a new solution to provide a board
base of
paperboard or cardboard intended to be used as packaging material with a
polymeric
layer of coating which renders the package liquid-tight and gas-tight. The
purpose is
particularly to provide a simple structure of coated board and savings in
coating
material, while at the same time making the coating tough enough to withstand
the
creasing required of paperboard or cardboard containers without breaking. The
invention is characterized by the steps of providing a polymerizing reaction
mixture
containing at least one silicon compound to form an inorganic, chain-like or
cross-
linked polymeric backbone containing alternating silicon and oxygen atoms, and
at
least one reacrive organic compound to form organic side chains and/or
crosslinks to
the polymeric backbone, spreading said mixture on the board base, and curing
said
mixture to form a layer of coating.
The process according to the invention can be implemented, starting from a
silicon
compound, such as silane, an organic compound reacting with it, water, and a
possible catalyst, whereby the hydrolyzed groups of the silicon compound are
first
partly condensed, forming colloidal particles in the solution. With the sol
ageing
and/or with a catalyst being added, the reaction continues with the particles
growing
and being combined, resulting in a chained or crosslinked gel covering the
surface
of the board, the gel being finally dried and cured by heating or irradiating
it using
UV, IR, laser or microwave radiation to form a thin, tight coating on the
board.
Depending on the circumstances, the curing time may vary from fractions of a
second to several hours. The coating thus obtained simultaneously features
typical
characteristics of both an inorganic and an organic substance, and the
properties of
the coating can be adjusted particularly by properly selecting the organic
component
which forms crosslinks or side chains.
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The organic compound as used is a purely organic carbon based compound capable
of forming organic, carbon based side chains or crosslinks. Through the
reactive
sites of the polymeric backbone formed by the silicon compound. Said organic
compounds are thus distinet from the silico-organic compounds such as
organosilanes which polymerize by hydrolysis and condesation of the alkoxy gl-
oups
into an essentially inorganic chain or network structure.
In the invention, a considerable portion of the polymeric layer can be formed
of
suitable reactive organic compounds which are essentially cheaper than organo-
silanes. Furthermore, an organic compound, which preferably is added to the
IO reaction mixture at a relatively late stage, advances the completion of the
polymerization. The polymeric backbone which is created when only organosilane
is used can constitute a steric hindrance to the mutual reactions of the
reactive
substituents of silane, while a free organic compound which is present is
presumably able to continue the reaction even after this, forming more side
chains
and/or crosslinks between the inorganic silicon-oxygen chains. By adjusting
the
amount of the organic compound used, the degree of organicity of the coating
thus
created and the properties depending on it can also be adjusted at the stage
of
polymerization.
According to the invention, an oxygen-tight and water vapor-tight and tough
layer
of coating is provided which does not break when bent, withstands creasing,
and
can be made very thin without creating small visually unperceivable pin holes
in the
coating, during the forming stage or later when heated or jointed, which
constitute a
problem in present coating materials and because of which the layers of
coating
have had to be made relatively thick. On the basis of preliminary tests, a
tight layer
of coating can be provided on a smooth board base by as low an amount of
coating
as 1 g/m2, and in practice, a preferred amount of coating is in the range of
about 2 to
6 g/m2. A further advantage is that a polymeric sealing layer can be spread
directly
on top of the silicon-based layer of coating without needing a binding agent
between
these layers. In known organic coating combinations, simply the weight of a
gas-
tight barrier which can be made of poiyanude, PET or EVOH is typically at
least
about 20 g/m2, and these materials require a separate layer of binding
material
between the barrier and the heat-seal layer. Therefore, the invention can be
used to
accomplish essential savings in material and a decrease in the weight of the
board as
compared with the said, known technology. Another advantage of the invention
is
that the spreading of the coating mixture is easy to accomplish using the
methods
commonly used in paper and paperboard or cardboard industry, such as rod or
blade
coating techniques or spraying. The spreading of the coating may thus be
effected in
the board machine by using the "on-line" principle as part of the
manufacturing
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process of the board, by using the same application technology that is used in
spreading normal coatings. The coating can also be effected on premoulded tray
blanks or in connection with the moulding. When needed, the mixture can be
extended with filling material, the most preferable materials including scale-
or
5 slate-like filling materials, such as talc, mica or glass flakes. When the
coating is
formed, these substances settle in the direction of the surface and contribute
to its
properties of impermeability. The adhesion of the coating to the filling
agents is
excellent. It is also possible to dye the coating by adding pigments or
organic
colouring agents to the mixture, or by mixing organic and/or inorganic fibres
or
particles into the coating formulation, the fastening of which to the coating
can be
improved by coupling agents. Furthermore, it is possible to include, in the
formulation, an organic, polymerizing agent which forms a separate polymeric
structure with respect to the inorganic chain or crosslinked structure,
according to
the invention, and which intermeshes with it. In addition to the board
machine, the
spreading of the coating can be carried out, in connection with a printing
process,
for example, on a finished board which does not necessarily have to be dried
first.
In this case, the board can be precoated with any kind of coating commonly
used in
paper and board industry.
The chain or crosslinked backbone of the polymeric coating provided according
to
the invention can consist of silicon and metal atoms and oxygen atoms which
alternate with them. Preferably, the structure mainly consists of silicon and
oxygen,
and fairly small amounts of metal atoms can be combined with the same
structure as
substituents for the silicon. The metals can preferably include, for example,
Ti, Zr,
and Al. The organic groups that are combined with the polymeric stl-ucture can
mainly include substituted or unsubstituted alkyl and aryl groups.
The polymerizing reaction that creates the inorganic polymeric backbone of the
coating, according to the invention, can be described by way of an example by
the
following formula:
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a Me(OR)4 + v (HX)nSi{OR)4_n + w (YX)mSi(OR)4_m -~
O ~ XY
+H20
--~ - O - Me - O - Si O - Si - O -
-HOR
(n=m=2)
O XH XY
a v w
in which:
Me refers to a tetravalent metal atom,
R refers to an alkyl group or hydrogen,
X refers to, for example, an alkyl or aryl body or chain,
Y refers to a reactive substituent which can be, for example, an amino, a
hydroxyl,
a carbonyl, a carboxyl, a vinyl, an epoxy or a methacrylate group,
u, v, and w are integer numbers, and
n and m are integers from 1 to 3.
In the organic polymerization of the coating composition which is preferably
carried
out at the drying and setting stage of the coating, an organic compound can
combine
with the reactive substituent Y of organosilane to form an organic side chain,
by
using an addition reaction. The reaction can also be a condensation depending
on
the reacting groups. The reactive group at the end of the chain can further
react with
substituent Y of organosilane in the polymerization, whereby an organic
crosslink is
created between the silicon chains. It is also possible that substituents Y of
organo-
silane react directly with each other to form a crosslink between the silicon
chains.
The number and the length of the crosslinks, i. e., the degree of organicity
of the
coating, can be adjusted with the aid of the quality and the fraction of the
organic
compound included in the reaction mixture. Particularly suitable crosslinking
organic compounds include epoxides which contain two epoxy groups in an alkyl
or
aryl body or chain, and diols.
The liquid medium needed in the process according to the invention may
contain,
for example, water, alcohol, and/or liquid silane. The hydrolyzation carried
out in
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the above reaction example binds water, providing that water is present, while
at the
same time alcohol is released in the reaction, converting into a liquid phase.
Organosilanes which comprise hydrolyzing and condensing groups, or their hydro-
lyzates are suitable for starting materials of the process according to the
invention.
Correspondingly, compounds containing metal centre atoms, such as Zr, Ti, Al,
B,
etc., or compounds of these metals and silicon, or mixtures of the compounds
can be
used. E.g. silanes of the following type can be used:
(YX)a(HX ~ )bsi(OR)4_a_b
in which
Y = a reactive organic group which is an epoxy group, a vinyl group or other
polymerizing, organic group,
X and X' = a hydrocarbon group containing 1 to I O carbon atoms,
R = a hydrocarbon group containing 1 to 7 carbon atoms, an alkoxyalkyl group
or
an acyl group containing 1 to 6 carbon atoms,
a = number 1 to 3,
b = number 0 to 2, providing that a + b < 3.
Organic polymerization can be described by way of an example in the following
way:
a) the reactive groups of the organosilane of the coating composition (Y in
the
above reaction equation) crosslink the coating when they are polymerized.
A polyethylene oxide crosslink formed by epoxy silane is presented as an
example:
2 ~ H CH cat. CH2 C _
~~
- CH O CH2
-Si-
Epoxy silane
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b) the added, organic, reactive prepolymer reacts with the reactive group of
the
organosilane
R
I
CH2 C O-
CH CH2 + CH CH2 cat'--~-
-CH O CH2
R
-S~.
Epoxy silane Epoxide
I
c) the added, organic, polymerizing substance reacts when the molecules of the
substance in question polymerize with each other
R
2 CH CHZ c~ /CH ~ ~C~ ~ -
-CH/ O \\CH2
R
Epoxide R
d) all alternatives a, b, and c can have an effect together.
The number and the length of the crosslinks, i. e., the degree of organicity
of the
coating can also be adjusted by the quality and the fraction of the organic
compound
included in the reaction mixtwe. The organic compound can be a monomer which
can be prepolymerized to a varying degree and/or combined with the silane at
the
time of spreading the mixture. The organic compound can also be in the foam of
a
prepolymer when added to the reaction mixture. The amount of the organic
compound can be, calculated as a monomer, 5 to 80, preferably 10 to 70, and
most
preferably 10 to 50 molar percent of the total amount of the polymerizing
starting
materials of the reaction mixture.
The epoxysilanes according to formula ( 1 ), containing one glycidoxy gl-oup
can
include, for example: glycidoxymethyltrimethoxysilane,
glycidoxymethyltriethoxy-
silane, ~i-glycidoxyethyltriethoxysilane, ~3-glycidoxyethyltrimethoxysilane,
'y-glycid-
oxypropyltrimethoxysilane, 'y-glycidoxypropyltriethoxysilane, y-
glycidoxypropyltri-
(methoxyethoxy)silane, y-glycidoxypropylh~iacetoxysilane, ~-glycidoxybutyltri-
methoxysilane, 8-glycidoxybutyltriethoxysilane,
glycidoxymethyldimethoxysilane,
glycidoxymethyl(methyl)dimethoxysilane, glycidoxymethyl(ethyl)dimethoxysilane,
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glycidoxymethyl(phenyl)dimethoxysilane, glycidoxymethyl(vinyl)dimethoxysilane,
(3-glycidoxyethyl(methyl)dimethoxysilane, ~3-
glycidoxyethyl(ethyl)dimethoxysilane,
y-glycidoxypropyl(methyl)dimethoxysilane, y-glycidoxypropyl(ethyl)dimethoxy
silane, 8-glycidoxybutyl(methyl)dimethoxysilane and b-glycidoxybutyl(ethyl)di
methoxysilane.
Silanes containing two glycidaxy groups can include, for example: bis-
(glycidoxy-
methyl)dimethoxysilane, bis-(glycidoxymethyl)diethoxysilane, bis-(glycidoxy-
ethyl)dimethoxysilane, bis-(glycidoxyethyl)diethoxysilane, bis-
(glycidoxypropyl)di-
methoxysilane, and bis-(glycidoxypropyl)diethoxysilane.
Examples of compounds according to formula ( 1 ), containing other reactive
groups
include: vinyltriethoxysilane, vinyl-tris((3-methoxyethoxy)silane,
vinyltriacetoxy-
silane, y-methacryloxypropyltrimethoxysilane, y-aminopropyltriethoxysiiane, N-
J3-
(aminoethyl)-y-aminopropyltrimethoxysilane, N-bis{/3-hydroxyethyl)-y-amino-
propyltriethoxysilane, N-(~3-aminoethyl)-y-aminopropyl{methyl)dimethoxysilane,
y-
chloropropyltrimethoxysilane, y-mercaptopropyltrimethoxysilane and 3.3.3-tri-
fluoropropyltrimethoxysilane.
Examples of silicon compounds which are described by general formula (2)
(HX)"Si(OR)4_" {2)
include dimethyldimethoxysilane, methyltrimethoxysilane, tetraethoxysilane,
phenyltrimethoxysilane and phenylmethyldimethoxysilane.
These compounds can be used as separate components or as mixtures of two or
more compounds.
Other possible compounds include, for example, colloidal silica, i. e., a
colloidal
solution which contains a certain fraction of very fine-grained silica-
anhydride
powder and which is dispersed in water or alcohol, for example, and in which
the
particle diameter is preferably 1 to 100 nm.
Prepolymers can be used as crosslinking organic compounds and the reactive
groups
of organosilanes preferably react with the prepolymers so that similar
reactive
groups react mutually to form crosslinks which combine inorganic oxygen
silicon
chains. For example, epoxide resin or aromatic diols can be used to react with
silanes containing epoxy groups.
Aromatic alcohols, such as Bisphenol A, Bisphenol S, and 1.5-dihydroxy
naphtalene can be used as diols. Acrylates can be used to react with silanes
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containing acrylic groups or acryloxy groups. Prepolymers which have reactive
double bonds are used with vinyl silanes or other silanes containing
polymerizable
double bonds, as well as with silanes containing mercapto groups. Polyols are
used
with silanes containing isocyanate groups. Isocyanates are used with silanes
5 containing hydroxy groups and epoxide resin is used with aminosilanes.
Mineral fillers, such as talc and mica can be used as filling material.
Furthermore,
coupling agents, tensides, and other additives which are used to prepare
composites
and coatings can be added to the mixture.
The hydrolyzates of the silicon compounds according to formulas ( 1 ) and (2)
can be
10 manufactured by hydrolyzing corresponding compounds in a solvent mixture,
such
as a mixture of water and alcohol in the presence of acid, of which the method
is
commonly known. When the silicon compounds according to general formula ( 1 )
and (2) are used in the form of hydrolyzates, a better result is generally
obtained by
mixing and hydrolyzing the silanes together.
A curing catalyst makes the coating cure at a relatively low temperature and
has an
advantageous effect on the properties of the coating.
The following substances, for example, can be used as the curing catalysts of
silanes
containing epoxy groups: Broensted acids, such as hydrochloric acid, nitric
acid,
phosphoric acid, sulphuric acid, sulphonic acid, etc.; Lewis acids, such as
ZnCl~,
FeCl3, AIC13, TiCl3, and the metal salts of these organo complex acids, such
as
sodium acetate, and zinc oxylate; organic esters of boric acid, such as methyl
borate
and ethyl borate; alkalis, such as sodium hydroxide and caustic potash;
titanates,
such as tetrabutoxy titanate and tetraisopropoxy titanate; metal acetyl
acetonates,
such as titanyl acetyl acetonate; and amines, such as n-butyl amine, di-n-
butyl
amine, guanidine, and imidazole.
Latent catalyzers can possibly also be used, such as salts of inorganic acids
and
carboxylic acids, such as ammonium perchlorate, ammonium chloride, and
ammonium sulphate, ammonium nitrate, sodium acetate, and aliphatic fluoro-
sulphonates.
The selection of the most suitable curing catalyst depends on the desired
properties
and the use of the coating composition.
Furthermore, the coating can contain solvents, such as alcohols, ketones,
esters,
ethers, cellosolves, carboxylates or their mixtures. Lower alcohols from
methanol to
butanol are particularly recommended. Methyl cellosolve, ethyl cellosolve, and
butyl cellosolve, lower carboxylic acids and aromates, such as toluene and
xylene,
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and esters, such as ethyl acetate and butyl acetate, are also commonly used.
However, the use of solvents is preferably minimized, for example, by using
silanes
as solvents because the evaporation of solvent vapors in connection with the
coating
of the paperboard causes extra arrangements.
To obtain a smooth coating, a small amount of a flow regulating agent (such as
block-copolymer of aikylenedioxide and dimethylsiloxane) can be added if
needed.
Antioxidants and substances which protect against UV-light can also be added
to
the coating.
Non-ionic tenside can be added to the coating solute to adjust its wetting
properties
l0 and hydrophilic properties.
The silicon-based coating layer provided according to the above description
has a
glassy outward appearance and it is also tight and flexible, does not crack or
form
holes, is heat-resistant and chemically resistant. The coating is oxygen-
tight, grease-
tight, aroma-tight, and water vapor-tight, and it is not sensitive to
moisture. In the
recycling of material carried out by pulping, the minor amounts of coating
material
present do not harm the recycled pulp thus obtained.
The curing of the coating layer and removing the remaining liquid phase are
preferably carned out by heating the coating to a temperature range of about
100 to
200 °C. Heating removes the porosity from the coating, giving it the
required liquid
tightness and gas-tightness.
As mentioned earlier, a joint-forming polymeric coating can be spread on top
of the
layer of coating provided according to the invention without a laminating
adhesive
between the layers. For example, when container-type packages are manufactured
from paperboard or cardboard, the heat-sealing polymer serves as an adhesive
that
seals the joints of the container. To ensure the tightness of the joints, both
sides of
the board are preferably coated with heat-sealing polymer.
As the thin, glassy coating layer provided according to the invention is
transparent,
the pictures and the text that have been printed on the board before the
coating
process will be visible. This is an advantage in food trays in which the
glassy
coating constitutes the outer surface of the product.
The coated packaging board manufactured according to the invention can be used
as
the oxygen-tight and aroma-tight material of containers or small cups intended
for
packages of liquid foodstuffs. The layer of coating withstands, without
breaking, the
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creasing of the coated paperboard to provide the edges of containers which
have the
shape of a rectangular prism or a tetrahedron.
Another special application of the packaging board coated according to the
invention is grease-tight, heat-resistant material of foodstuff bases, such as
micro-
s wave or conventional oven trays. In this case, too, the paperboard is
subjected to
creasing and folding and the coating must withstand the treatment without
breaking.
Furthermore, one special advantage of the coating of ovenable trays, provided
according to the invention, is the good heat-resistance of the coating. The
paper-
board can be shaped into a tray by pressing at a high temperature and the
trays
easily withstand the normal temperatures of kitchen stoves and microwave
ovens,
and even temperatures exceeding 300 °C at which the paperboard will
begin to char.
At the same time, the layers of coating protect the paperboard from the
softening
effect of steam coming from the food when heated so that the tray maintains
its
fol-m. When baked, the food does not stick to the coating according to the
invention.
The nay provided in accordance with the invention can be part of the consumer
packaging of prepared food, for example, whereby the food is intended to be
heated
in the tray after opening the package, or the trays can be sold to consumers
as such.
Furthermore, the invention comprises a method for manufacturing a liquid-tight
and
gas-tight package, which is characterized in that a polymerizing reaction
mixture is
spread on paper or a board base of paperboard or cardboard, said mixture
comprising at least one silicon compound forming an inorganic, chain or
crosslinked
polymeric backbone which contains alternating silicon and oxygen atoms, and at
least one reactive, organic compound forming organic side chains and/or
crosslinks
to the polymeric backbone, that the reaction mixture is cured to form a layer
of
coating, and that the package is partly of fully formed of the thus obtained
polymer
coated paper or board.
It should be mentioned in this context that the board base in the present
invention
refers to a fairly stiff fibre-based packaging material which is sufficiently
self
supporting to be suitable for container-like packages or foodstuff bases, for
example, which are manufactured entirely of the said material. The weight of
such a
board is at least about 170 g/m2, and generally in the order of 225 g/m2 or
higher. A
board in the weight range of 170-250 g/m2 is conventionally referred to as
paper-
board and a board having a weight of 250 g/m2 or more is referred to as
cardboard.
The paper in the invention refers to a thinner and lighter fibre-based
material which
is suitable packaging material, for example, for heat-sealed, peelable covers
of
portion packages or boxes.
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13
What is presented above in connection with the manufacturing method of
packaging
board according to the invention, is mainly applicable as such to the
manufacturing
method of the package according to the invention. This is related, for
example, to
the forming of the silicon-based layer of coating, its chemical structure and
composition, and to a possible spreading of a jointing polymeric coating on
top of
the glassy silica coating.
Products according to the invention, manufactured according to the methods
described above, include particularly sealed paperboard and cardboard packages
of
liquid foodstuffs, such as milk, cream, sour milk or juice containers and
small cups,
sealed paper foodstuff packages, such as soup mix powder pouches, coffee, and
spice packages, paperboard microwave or conventional oven food trays which can
be part of prepared food packages, paperboard or cardboard detergent packages,
and
the heat-sealed paper covers of glass, plastic or paperboard foodstuff,
medicine, and
cosmetics packages, particularly the covers of yoghurt, milk, juice, water,
ice-cream
or dessert cups, and those of curdled milk containers or butter, margarine or
prepared foodstuff boxes.
In the appended drawing which illustl-ates the products according to the
invention,
Fig. 1 shows a small yoghurt cup according to the invention which is provided
with a heat-sealed cover paper,
Fig. 2 is a section of the mouth of the cup and the edge of the cover paper as
a
part enlargement of Fig. 1,
Fig. 3 shows a paperboard ovenable tray according to the invention,
Fig. 4 is a section of the edge of the tray as a part enlargement of Fig. 3,
Fig. 5 shows a paperboard milk container according to the invention, and
Fig. 6 is a section of the wall of the container as a part enlargement of Fig.
5.
The consumer package of yoghul-t according to the invention presented in Figs.
1
and 2 preferably consists of small plastic cup I with an oxygen-tight and
aroma-
tight cover paper 3 heat-sealed on its mouth 2. Cover paper 3 comprises paper
layer
4, silicon-based, oxygen-tight and aroma-tight polymeric layer 5 made by using
a
sol-gel process, and, for example, heat-seal layer 6 of styrene-modified
copolymer
of ethylene and methacrylic acid. Heat-seal layer b secures cover paper 3
tightly to
flange 2 encircling the mouth of the pot. At the same time, heat-seal layer 6
allows
cover paper 3 to be peeled when the cup is opened. The weight of paper layer 4
of
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14
the cover paper can be, for example, 40 to 80 g/m2, the weight of the oxygen-
tight
and aroma-tight layer of coating 5 is preferably about 2 to 5 g/m2, and the
weight of
heat-seal layer 6 can be, for example, about 20 g/m2.
Ovenable tray 7 according to Figs. 3 and 4 which can be applied to a package
of
prepared food, for example, comprises paperboard layer 8 and glassy, silicon-
based
polymeric layers 9 formed by a sol-gel process on the inner and outer surfaces
of
the tray. The weight of the paperboard layer is at least about 225 g/m2, and
the
weight of both glassy polymeric layers 9 is preferably about 2 to 5 g/m2.
Polymeric
layers 9 render the tray water-tight and grease-tight and they withstand the
conventional kitchen stove operating temperatures of 200 to 250 °C
without being
damaged. The polymeric layer of the inner surface of the tray specifically
prevents
the food from sticking and the polymeric layer of the outer surface of the
tray
mainly protects the tray against the grease on the bake sheet and against the
splatters
coming from the food when heated. In some instances, the polymeric layer of
the
tray outer surface can be omitted. The illustrated tray 7 as such can also be
used in
microwave ovens.
Milk container 10 which is illustrated in Figs. 5 and 6 and which is mainly
shaped
as a rectangular prism is made entirely of coated, liquid-tight and gas-tight
packaging board. The packaging board comprises a polymeric heat-sealing layer
11
on the outer surface of container 10, paperboard layer 12, a silicon-based,
oxygen-
tight and aroma-tight polymeric layer I3 made by a sol-gel process and placed
inside of the paperboard layer, and a heat-sealing layer 14 which constitutes
the
inner surface of the container. Heat-sealing layers 11, 14 of e.g.
polyethylene at the
joints of container 10 secure the overlapping paperboard layers tightly to
each other.
The weight of paperboard 12 of the container is at least about 225 g/m2, the
weight
of the oxygen-tight and aroma-tight polymeric layer 13 is preferably about 2
to 5
g/m2, and the weight of both heat-sealing layers I 1, 14 is, for example,
about 20
2
g/m .
The packaging board according to Fig. 6 which constitutes the wall of the
container
can be provided with an extra polymeric layer (not shown) between paperboard
layer 12 and sol-gel layer 13 which possibly also contains pigments and
fillers.
Examples of preferred polymers include polyolefms and styrene acrylates. The
said
polymeric layer can be used to decrease the material thickness of sol-gel
layer 13
because the polymeric surface is smoother and tighter than the paperboard
layer.
The invention and the polymeric coating materials it employs are described by
the
following application examples.
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Example 1 Barrier coating
182 g of 2.2-bis(4-hydroxyphenyl) propane is dissolved by mixing in 473 g of
gamma-glycidyloxypropyltl-imethoxysilane at room temperature. 24 g of 0.1 N
hydrochloric acid is gradually added to this mixture, agitating it at the same
time.
5 Agitation is continued for about two hours, during which time about 20 g of
colloidal silica is added. When needed, 1 g of a flow regulating agent is
added. The
solution thus prepared is usable for at least one month. 16 g of methyl
imidazole (a
Lewis acid) is added by mixing for about one hour before the solution is used.
This
solution is usable for about 24 hours.
1 o The coating was effected by using the rod coating method on the following
paperboard grades:
1. Pigment coated SBS paperboard
Basis weight 235 g/m2
Thickness 314 p,ln
15 2. Styrene butadiene dispersion coated paperboard
3. Cup board with smooth surface
Basis weight 230 g/m2
Thickness ~ 300 pm
The coating was heat-cured in a furnace at 160 °C for 2 minutes.
Test results
The coating solution according to Example 1 was used in the tests conducted on
paperboard grades 1, 2, and 3. The results indicate that the coating solution
with this
viscosity suited smooth and less porous paperboard grades the best (samples I
and
2).
When assessed visually, the coating is clear, transparent, and it has a good
film
forming ability. On the basis of an electron microscope study, the coating in
samples 1 and 2 is whole and continuous. The coating in sample 3 is partly
absorbed by the pores, causing holes.
The physical properties of the coating are shown in Table 1.
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Table 1
The test results of Example 1
PaperboardThicknessPenetrationPeneh~ationResistanceResistance
grade of coatingof water of oxygen to oil to
and
p.m vapor cm~/m2/24 grease, temperature,
KIT-
g/m2/24 h, 23 C TEST DSC 25-
h,
23 C, 50% 300 C
RH
1. Pigment5 9 23 12 No changes
SBS
2. 4 3 30 12 No changes
Dispersion
coated
3. Smooth 6 25 420 8 No changes
cu board
Example 2
The solution is prehydrolyzed as in Example 1.
Instead of colloidal silica, small amounts of fine-grained talc, totalling 180
g, are
added by agitating continuously, 98% of the grain size of the talc being less
than 10
pm (Finntalc C 10).
After methyl imidazole had been added to the mixture, its viscosity was
adjusted to
suit the rod coating by adding about 7 g of colloidal silica to it.
The coating solution was used to coat paperboard grades l and 3 according to
Example 1. The coating was hardened and dried in the same conditions as in
Example 1.
Test results
When assessed visually, the coating is slightly matte and it has a good film
forming
ability.
The physical properties of the coating are presented in Table 2.
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Table 2
The test results of Example 2
PaperboardThickness PenetrationPenetrationResistanceResistance
of
grade coating of water of oxygen to oil to
~m and
vapor cm3/m2/24 grease temperature
h
g/m2/24 KIT-TEST DSC 25-
h
300 C
1. Pigment10 11 33 12 No changes
SBS
3. Smooth 12 9.8 29 12 No changes
cup board
Example 3
35.6 g of phenyltrimethoxy silane, 276.6 g of
glycidyloxypropyltrimethoxysilane
and 19.8 g of aminopropyltriethoxysilane were mixe
d in a vessel in an ice bath. 6 g of water was gradually added to this mixture
by
dropping and agitation in the ice bath was continued for 15 minutes, whereupon
12
g of water was added in small amounts and the mixture was further agitated in
the
ice bath for 15 minutes. Then 97.2 g of water was added by dropping it faster
and
agitation was continued for two hours at room temperature. Then 43.6 g of
epoxy
resin (Dow Corning D.E.R. 330) was added to this hydrolyzate. Coating was carl-
ied
out on paperboards 1 to 3 according to Example 1 by using the rod coating
method.
The coating was cured in a furnace at 160 °C for three minutes.
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Table 3
The test results of Example 3
PaperboardThickness PenetrationPenetrationResistanceResistance
of
grade coating of water of oxygen to oil to
p.m and
vapor cm~/m2/24 grease temperature
h
g/m2/24 23 C KIT-TEST DSC 25-
h
23 C, 50% 300 C
RH
1. Pigment4 10 25 12 No changes
SBS
2. 4 4 32 12 No changes
Dispersion-
coated
3. Smooth 6 12 35 12 No changes
cu board
Example 4
The solution was prehydrolyzed as in Example 3. 147 g of mica (Kemira Mica 40)
was added to the hydrolyzate. The coating solution was used to coat the
paperboard
grades 1, 2, and 3 according to Example 1. The coating was cured and dried as
in
Example 3.
Test results
l0 When examined visually, the coating is slightly matte and it has a good
film forming
ability. The physical properties of the coating are presented in Table 4.
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19
Table 4
The test results of Example 4
Paperboard Thickness PenetrationPenetrationResistanceResistance
of
grade coating of water of oxygen to oil to
pm and
vapor cm~/m2/24 grease temperature
h
g/m2/24 23 C KIT-TEST DSC 25-
h,
23 C, 50% 300 C
RH
1. Pigment 5 8 20 12 No changes
SBS
2. ( 4 25 12 No changes
Dispersion
coated
3. Smooth 6 10 30 12 No changes
cu board
It is clear to those skilled in the art that the different embodiments of the
invention
are not limited to the examples described above but can vary within the
appended
claims.
